In all secondary and tertiary enhanced oil recovery processes in which a drive fluid is used to force oil from an injection well to a production well, profile control may be needed to overcome the deleterious effects of formation permeability stratification. When formation permeability inhomogeneity is encountered, injected and driven fluids preferably travel through the most permeable regions, resulting in low sweep efficiency and bypassing of oil in lower permeability regions.
Another problem encountered is that of gravity override. This occurs when injected fluids much lighter than the reservoir fluids separate by gravity, rising toward the top of the flowing region which results in bypassing of lower regions. One commonly employed solution to these problems is to inject a material that will preferentially flow into the more permeable regions (or is selectively introduced into higher regions) but will subsequently impede further flow through them, thus diverting flowing fluids into previously uninvaded regions. Polymer gels, or polymer/gelling agent mixtures that will subsequently gel in situ have been used.
The enhanced oil recovery techniques of waterflooding, carbon dioxide flooding, miscible or immiscible gas flooding and steam flooding are of particular interest and importance. Profile control can often improve performance in such processes by reducing the effects of permeability stratification or gravity override. A gel suitable for profile control must form and be stable enough to continue to impede flow for long periods of time at the reservoir temperature, salinity and pH. A variety of materials are commercially available for profile control, all of which perform differently and have limitations. Biopolymers such as xanthan gums are unstable above about 140.degree. F. Synthetic polyacrylamides, depending upon their degree of hydrolysis and the nature and amount of other functional groups such as alkyl sulfonate or pyrrolidone, will have a temperature above which they will not be useful at a given salinity. Depending upon the specific conditions in a reservoir, one or more of these will be favored on the basis of cost-performance considerations.
Metal-complexed polymer gels are widely used for profile control. For example, Al (III) and Cr (III) are used to cross-link polyacrylamide. Other metal ions, such as Ti (IV), Zr (IV), Fe (III), etc., are also useful as gelants for polymers.
Several limitations may interfere with the use of metal ions in the preparation of gels for profile control. One limitation is that each metal is reactive only to certain functionalities. For example, Al, Cr and Zr are reactive to amide and carboxyl groups, while Ti is reactive to hydroxyl groups. A proper match of the polymer with the appropriate metal cross-linker must be considered. There is no presently known general metal cross-linker for all polymer types. Carbonate, biocarbonate and sulfate anions are known to interfere with the gelation of Cr, Zr and Al. Another limitation is that pH control is important for most metal cross-linking reactions. It is easy to control the pH when the gel is prepared above surface but difficult to do when an in-situ gelation processed is used. Furthermore, ligand-metal bond formation and stability may be affected by high ionic strength and the temperature of reservoir brine. At substantially high brine concentrations and high temperatures metal-ligand bonds can dissociate due to unfavorable equilibria.
Therefore, what is needed is an optimal range of economical amino resins which will co-gel and cross-link covalently with all polymers known to be useful to profile control where said polymers contain amine, amide, hydroxyl or thiol functionalities, thereby forming more stable gels. The utilized gelation reaction should proceed under all pH conditions; should not require an acid or base to catalyze; and should not be affected by reservoir brine. The resultant linkages should be stable at substantially high temperatures and high salinities.